1. Field of the Invention
The present invention relates generally to vehicle guidance, and in particular to a system and method for global navigation satellite system (GNSS)-based positioning using a base station to provide differential GNSS (DGNSS) data for real time kinematic (RTK) vehicle guidance, wherein the base station communicates with the vehicle receiver and the vehicle receiver reallocates the satellites and the satellite systems it is tracking to match those tracked by the base station.
2. Description of the Related Art
GNSS guidance and control are widely used for vehicle and personal navigation and a variety of other uses involving precision location in geodesic reference systems. GNSS, which includes the Global Positioning System (GPS) and other satellite-based positioning systems, has progressed to sub-centimeter accuracy with known correction techniques, including a number of commercial satellite based augmentation systems (SBASs).
DGNSS uses a localized base receiver of known location in combination with a rover receiver on a moving vehicle for obtaining accurate vehicle positions from GNSS data. Differential positioning, using base and rover receivers, provides more accurate positioning information than standalone systems because the satellite ranging signal transmission errors tend to affect the base and rover receivers equally and therefore can be cancelled out in computing position solutions. In other words, the base-rover position signal “differential” accurately places the rover receiver “relative” to the base receiver. Because the “absolute” geo-reference location of the fixed-position base receiver is precisely known, the absolute position of the rover receiver can be computed using the known base receiver absolute position and the position of the rover receiver relative thereto.
For even more accurate information, higher frequency signals with shorter wavelengths are required. It is known in the art to use GNSS satellites' carrier phase transmissions and carrier phase signal components from base reference stations or SBAS satellites, including the Wide Area Augmentation System (WAAS, U.S.), and similar systems such as EGNOS (European Union) and MSAS (Japan). With such augmentation, a position may readily be determined to within millimeters. When accomplished with two antennas at a fixed spacing, an angular rotation may be computed using the position differences.
An example of a GNSS is the Global Positioning System (GPS) established by the United States government, which employs a constellation of 24 or more satellites in well-defined orbits at an altitude of approximately 26,500 km. These satellites continually transmit microwave L-band radio signals in two frequency bands, centered at 1575.42 MHz and 1227.6 MHz, denoted as L1 and L2 respectively. These signals include timing patterns relative to the satellite's onboard precision clock (which is kept synchronized by a ground station) as well as a navigation message giving the precise orbital positions of the satellites, an ionosphere model and other useful information. GPS receivers process the radio signals, computing ranges to the GPS satellites, and by triangulating these ranges, the GPS receiver determines its position and its internal clock error.
In standalone GPS systems that determine a receiver's antenna position coordinates without reference to a nearby reference receiver, the process of position determination is subject to errors from a number of sources. These include errors in the GPS satellite's clock reference, the location of the orbiting satellite, ionosphere induced propagation delay errors, and troposphere refraction errors.
To overcome these positioning errors of standalone GNSS systems, many positioning applications have made use of data from multiple GNSS receivers. Typically, in such applications, a reference or base receiver, located at a reference site having known coordinates, receives the GPS satellite signals simultaneously with the receipt of signals by a remote or rover receiver. Depending on the separation distance between the two GPS receivers, many of the errors mentioned above will affect the satellite signals equally for the two receivers. By taking the difference between signals received both at the reference site and the remote location, these errors are effectively eliminated. This facilitates an accurate determination of the remote receiver's coordinates relative to the reference receiver's coordinates.
The technique of differencing signals from two or more GPS receivers to improve accuracy is known as differential GPS (DGPS). Differential GPS is well known and exhibits many forms. In all forms of DGPS, the positions obtained by the end user's remote rover receiver are relative to the position(s) of the reference base receiver(s). GPS applications have been improved and enhanced by employing a broader array of satellites such as GNSS and WAAS. For example, see Whitehead et al. U.S. Pat. No. 6,469,663 for Method and System for GPS and WAAS Carrier Phase Measurements for Relative Positioning, which is assigned to a common assignee herewith and is incorporated herein by reference.
Although the above-mentioned GNSS networks such as EGNOS (European Union), MSAS (Japan), GPS (U.S.A.) and GLONASS (Russia) are available, traditional guidance systems are designed to function on only one of these GNSS systems at a time. This means that the potential to increase the positioning accuracy of a guidance system exists by allowing a guidance receiver to receive positional information from satellites belonging to more than one satellite network.
Heretofore there has not been available a DGNSS system and method for optimizing position information of a moving vehicle tracking multiple different GNSS networks using a single receiver unit.